1. Field of the Invention
The present invention relates to a bar code reading apparatus which emits a laser beam for scanning a normal plane to the plane of a read window in multiple directions outward from the read window and reads out a bar code moving outside the read window.
2. Description of the Related Art
According to conventional bar code reading apparatuses, particularly, those of a counter type, generally a laser beam emitted from a laser beam source disposed in the casing enters a converging lens, after reflected by multiple optical-path change mirrors, to be converged into a small beam spot at a predetermined focus position. Then, the laser beam scans a given object while simultaneously being reflected by a rotational scan mirror, and is then reflected by a plurality of fixed scan mirrors, thus becoming multiple scan beams, which are in turn irradiated outward from the read window. Conventionally, the scan beams are normally emanated as five groups of beams, and when any one of the beams crosses the bar code affixed to an object moving outside the read window, modulated reflection light produced there enters a light-receiving lens through the same path as used for emission of the laser beam, and is then received at a light-receiving element.
According to the above conventional bar code reading apparatus, as disclosed in, for example, U.S. Pat. No. 4,799,164 and U.S. Pat. No. 4,861,973, there are five groups of scan beams which appear on the read window and the normal plane. Referring to FIG. 1, there are five groups of scan beams which appear on a read window plane 13 and a normal plane 14: one group consisting of horizontal scan beams B.sub.1H and B.sub.1V, two groups including oblique scan beams B.sub.2H, B.sub.2V, B.sub.3H and B.sub.3V, and two more groups having vertical scan beams B.sub.4H, B.sub.4V, B.sub.5H and B.sub.5V. Each group of scan beams consists of two approximately parallel scan beams since the rotational scan mirror has two reflecting surfaces formed at slightly different angles. For the five groups of scan beams, therefore, there are ten scan beams produced. As the optical system uses many optical components, such as mirrors and lenses, the bar code reading apparatus inevitably becomes large, resulting in an increased manufacturing cost.
There is known a conventional bar code reading apparatus designed to be compact as disclosed in Published Unexamined Japanese Patent Application No. 64-48017, and this apparatus has a structure as shown in FIGS. 2 and 3.
According to this type of bar code reading apparatus, as shown in FIG. 2, a laser beam from a laser beam source 21 travels through a collimation 22, is reflected by optical-path change mirrors 23 and 24, passes a hole 25a of a perforated lens 25, and is reflected by a polygon rotational mirror 26 for scanning. The scan laser beam is reflected by a single fixed horizontal scan mirror 27 to become a group of horizontal scan beams, which are in turn emitted outside through a read window 30. The scan laser beam is also reflected by two fixed oblique scan mirrors 28.sub.1 and 28.sub.2, becoming two groups of oblique scan beams, which are also irradiated outside through the read window 30. The scan laser beam is further reflected by two fixed vertical scan mirrors 29.sub.1 and 29.sub.2, providing two groups of vertical scan beams, which in turn leave outside through the read window 30.
As the scan beams acquired by reflection at the polygon rotational mirror 26 are reflected once by the respective fixed scan mirrors 27, 28.sub.1, 28.sub.2, 29.sub.1 and 29.sub.2 to provide scan beams traveling in the respective directions, the number of optical components is reduced, thus contributing to realizing the bar code reading apparatus compact and reducing its manufacturing cost.
It is known that the longer the scan lines or the shorter the time to produce a series of scan lines, the better or higher the reading performance of this type of bar code reading apparatus.
Since the length of the scan lines is proportional to the distance to a bar code from the scan center of the polygon rotational mirror, however, it is difficult to make the scan lines longer for the conventional compact bar code reading apparatus. The time to produce a series of scan lines can of course be shortened by increasing the rotational velocity of the polygon rotational mirror. However, this method also increases the linear velocity of the scan beams, so that a decoder for decoding binary information acquired from the output of a light-receiving element, which receives reflection light from the bar code, cannot follow the increased speed. In this respect, shortening the time to generate a series of scan lines is limited.
In short, it is difficult to enhance the reading performance the conventional compact bar code reading apparatus.